The worldwide buildup of integrated communication networks for broad-band applications creates ever more switching centers for switching through different types of digital data in the ATM--Asynchronous Transfer Mode, according to a CCITT recommendation. The actual ATM switching unit receives the randomly arriving, marked data cells at a number of its inputs, marks them again and guides them to the proper one of its many outputs, so that the respective data cells arrive at their destination as quickly as possible. If the traffic is too high, the loss of cells and thereby the loss of data can occur, in spite of buffer memories. A switching unit must therefore be tested to determine whether it can handle the specified load.
The current means for load testing are so-called benchmark tests, which are defined, specially generated input signals that presuppose certain results. Deviations provide information about the functional ability of the test sample. Because of the large number of in- and outputs, the individual control of all inputs and the measurement of all outputs is not economically feasible--each input to be addressed requires a transmitting unit, each output to be measured requires a receiving unit. It is therefore commonplace to test only one or a few in- and outputs with the benchmark signal. In order for a baseload to be nonetheless available, at least one other input receives a baseload signal from a separate generator. It is marked so that it gets switched from an input to the output with the same number. Outputs that do not take part in the bench mark test are fed back to the next input, so that the baseload successively passes through all of these in- and outputs. This simulates additional traffic. However, the process has the disadvantage that errors which occur e.g. as a result of overload or intentional input, continue in batches and it becomes difficult to maintain defined conditions that allow the test signal to be evaluated.